Aromatic polyesters generally are semi-crystalline and have low melt strength. Containers made from polyethylene terephthalate (PET), with minor amounts of a comonomer, by the injection stretch molding process (ISBM) are the most common transparent container on the market. However the ISBM process is limited to uniform shapes and cannot produce bottles with a handle. Handles are desirable for larger bottles and containers for easy of handling by the consumer. Such larger bottles and containers with handles can be produced by the extrusion blow molding (EBM) process.
A typical extrusion blow molding manufacturing process involves: a) melting the resin in an extruder; b) extruding the molten resin through a die to form a tube of molten polymer (a parison); c) clamping a mold having the desired finished shape around the parison; d) blowing air into the parison, causing the extrudate to stretch and expand to fill the mold; e) cooling the molded container; f) ejecting the container from the mold; and g) removing excess plastic (flash) from the container (if any).
The hot parison that is extruded in this process often must hang for several seconds under its own weight prior to the mold being clamped around it. During this time, the extrudate must have high melt strength. A material with high melt strength can resist stretching, flowing, and sagging, that would cause uneven material distribution in the parison and thinning of the parison walls. The sag of the extruded parison is directly related to the weight of the parison, whereby larger and heavier parisons will have a greater tendency to sag. Melt strength is directly related to the viscosity of the polyester resin at the temperature of extrusion from the die, at zero shear rate. However a resin with high melt strength, or high melt viscosity at zero shear rate, is too viscous to be extruded in the extruder and pumped through the die without using high temperatures which cause the polymer to degrade and lose its melt viscosity. The polyester resin must have a rheology such that the viscosity at the shear rates associated with the extrusion process, generally 100 to 1000 s−1, is lower than the viscosity at zero shear rate, i.e. exhibits shear thinning.
The typical PET resins used to ISBM beverage containers are not suitable for extrusion blow molding due to their relatively low intrinsic viscosities (IV≦0.85 dL/g) and high melting points (>245° C.) which gives a low melt strength at the temperatures needed to process them.
In addition during the extrusion blow molding process the molten polyester cannot thermally crystallize otherwise a cloudy container is produced. Unlike the ISBM process, the EBM process produces waste from the flash that has to be cut off the molded container where, for instance, it has been clamped. This waste from the EBM process must be reground and mixed with the virgin resin and dried prior to re-extrusion.
Prior art has met these requirements for extrusion blow molding by using comonomers such as isophthalic acid, 1,4-cyclohexanedimethanol in order to reduce the thermal crystallization rate (Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters 2003, 246-247). Amorphous copolyesters using 1,4-cyclohexanedimethanol as a comonomer for EBM have been disclosed, for example in U.S. Pat. Nos. 4,983,711; 6,740,377; 7,025,925; 7,026,027; 2006/0094858; 2009/0181196; 2009/01812017,834,127; 7,915,374 and 2001/0081510. Higher melt strengths at a zero shear rate with shear thinning that reduces the melt viscosity at higher shear rates have been achieved by the use of branching agents such as trimellitic anhydride and pentaerythritol as disclosed in U.S. Pat. Nos. 4,132,707 and 4,999,388. All these copolyesters designed for EBM are essentially amorphous copolyesters. Alternatively high melt strength copolyester with an ultra-high molecular weight (IV>1.1 dL/g) can be used to provide the necessary melt strength as they exhibit some degree of shear thinning (US 2011/0256331). These ultra-high IV polyester resins have to be processed at higher temperatures which cause the resin to thermally degrade giving increased yellowness in the container, and a narrow EBM processing window.
Containers made from amorphous copolyesters, when added to the postconsumer PET recycling stream, tend to cause sticking, agglomeration and bridging issues during the drying process. This makes such EBM PET resins unsuitable for reuse in the post-consumer polyester recycle stream that is used in blends with virgin resins for use in the standard container and bottle ISBM process. PolyClear® EBM resin (Auriga Polymers Inc., Spartanburg, S.C. USA) is one commercial resin (partially crystalline) that has been approved by the Association of Postconsumer Plastic Recyclers (APR) for recycling in the postconsumer recycling stream.
A key requirement for an extrusion blow molded container is its ability to be dropped with a liquid therein without breaking. It is well known that amorphous polyesters age with time (A. Bhakkad, E. A. Lofgren and S. A. Jabarin, ANTEC 2000 Conference Proceedings, pages 2019-2023) making containers made from amorphous polyesters more brittle with age (lower impact resistance), and thus more prone to breakage when dropped.
Fillers have been added to polyester film and bottle resins in order to reduce the coefficient of friction of the film and/or bottle surface, antistick or antiblock agents. Antislip agents based on silicas have been disclosed in U.S. Pat. Nos. 5,278,205; 5,278,221; 5,384,191; 5,266,397; 5,281,379; 5,475,046; 5,382,651 and 6,323,271. Antiblock agents based on talc, tethered talc with fatty acids, barium sulfate, zinc sulfate and calcium phosphates are disclosed in U.S. Pat. No. 6,903,154 directed at reducing the coefficient of friction of ISBM polyester bottles, while maintaining clarity. US2011/0150751 discloses typical antiblock agents for polyester films: calcium carbonate, amorphous silica, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, lithium fluoride, the calcium, barium, zinc or manganese salts of the dicarboxylic acids used, carbon black, titanium dioxide, kaolin or crosslinked polystyrene particles or crosslinked acrylate particles. The antiblocking agents selected may also be mixtures of two or more different antiblocking agents or mixtures of antiblocking agents of the same composition but different particle size.
None of these patents discloses the use of these fillers to improve the aging of polyester articles.
There is therefore a need for a polyester resin that meets all the requirements of the extrusion blow molding process to form a container that maintains its impact resistance over time.